1. Field of the Invention
The present invention relates to a piezoelectric device including a thin film of a piezoelectric single crystal and a method for manufacturing the piezoelectric device.
2. Description of the Related Art
Currently, there are various piezoelectric devices including single-crystalline piezoelectric substrates. In particular, there are surface acoustic wave and plate wave devices including IDT (Interdigital Transducer) electrodes (also referred to as comb-shaped electrodes). Plate waves are those which have vibration displacements on both surfaces of an elastic substance and which propagate along the surfaces. An example of the plate waves is shown in, for example, FIG. 1.19 of Danseiha Soshi Gijutsu Handobukku. Among plate waves propagating in anisotropic crystals such as LiTaO3 and LiNbO3 single crystals are not only pure SH waves (a u2 component only) and Lamb waves (a u1 component and a u3 component only) but also modes propagating with displacement components such as u1 u2, and u3 being combined (that is, SH waves having a slight u1 component and a slight u3 component and Lamb waves having a slight u2 component).
A conventional piezoelectric device including such IDT electrodes has, for example, a structure disclosed in Japanese Unexamined Patent Application Publication No. 2007-251910. In particular, the conventional piezoelectric device includes a composite piezoelectric substrate including a piezoelectric thin-film and a support bonded thereto and also includes IDT electrodes formed on a surface of the piezoelectric thin-film that is located opposite to the support. In order not to inhibit the vibration of a region of the piezoelectric thin-film that has the IDT electrodes formed therein, a space is formed on the side of bonded surfaces of the piezoelectric thin-film and the support so as to correspond to the IDT electrode-formed region.
However, the conventional piezoelectric device, which includes the IDT electrodes as disclosed in Japanese Unexamined Patent Application Publication No. 2007-251910, is configured such that the IDT electrodes are exposed at the surface of the conventional piezoelectric device. Therefore, there are various problems as described below.
In back-end steps of a process for manufacturing a module for mounting the piezoelectric device, metal powders and the like may adhere to a surface thereof. This causes short circuits between electrode fingers of the IDT electrodes. If an insulating layer made of SiO2 or SiN is formed on an IDT electrode-formed surface for the purpose of preventing such a short-circuit failure, the vibration of the piezoelectric thin-film will be inhibited by the insulating layer.
If organic contaminants adhere thereto, gases will possibly be generated from the contaminants because of the thermal history of the back-end steps. The generated gases corrode the IDT electrodes to vary the excitation mode thereof and therefore adversely affect properties thereof. Particularly, in a sensor which includes IDT electrodes and which operates to detect the mass applied to a piezoelectric thin-film portion (membrane) not supported by a support, such corrosion leads to the deterioration of detection performance and therefore seriously reduces functions of the sensor.
In particular, for a plate wave device, the frequency thereof is significantly affected by the thickness of a piezoelectric thin-film defining a membrane. Therefore, when the piezoelectric thin-film is polished for frequency adjustment, the IDT electrodes are also ground or milled, whereby properties thereof are deteriorated. Likewise, when the thickness of the piezoelectric thin-film is adjusted by ion milling, the electrodes are ground or milled. When the membrane has an opening facing a support, a surface of the piezoelectric thin-film that faces a surface having the IDT electrode can be polished or milled. However, a boundary portion near the support cannot be polished or milled to a uniform thickness because a recessed portion is ground or milled. This causes differences in properties.